This invention is directed to the cancellation of clutter surrounding moving objects in radar applications preserving a usable monopulse ratio to be formed using adaptive processing techniques, and allowing both detection and angle location determination of objects in the radar image.
The problem of canceling clutter surrounding an object to be resolved in a radar image is a constant struggle for all radar applications. Specifically, clutter arises often times in space borne or in airborne radar systems when imaging the surface of the earth. In these applications the ground clutter must be suppressed for target detection, and the angular location of the object must be determined accurately to enable tracking of the object. Commonly, space-time adaptive processing (STAP) is utilized for target detection, where a sum beam is adaptively beam formed. However in order to perform target tracking, or angle location using a monopulse ratio, a ratio of adaptive difference beam to adaptive sum beam needs to be determined, which requires the further calculation of a difference beam. Unfortunately, as a result of the commonly used STAP processing, any monopulse ratio that could be developed would be distorted by the adaption due to mainlobe clutter. This ratio is so distorted by the clutter that it cannot be used for target angle location determination.
There is a keen interest in space-based radar platforms for large area surveillance. A space-based platform is able to provide a much greater coverage area than the traditional airborne platforms. For example space-based platforms have unlimited access to any location on the earth whereas airborne platforms may not. The main challenge to any of these systems has been the elimination of the ground clutter for detection and location of targets. This problem, which is present in all radars, is then heightened when the relative velocities of the object to the space-borne platform is great. This relative velocity is extremely large when the radar system is in orbit around the earth traveling at around 7500 m/sec and the object being detected and tracked is on the earth traveling at around 20 MPH. In such instances the object being detected and tracked is often completely obscured by ground clutter and it is very difficult to form a useable radar image of the object. Additionally, there is the possibility that the object itself is actively transmitting a jamming signal to prevent its discovery. Both the ground clutter and the jamming signal have some value of jamming power (J) associated with them. These are distinguished by one being actively produced while the other is a product of environmental factors. However, their effect is essentially the same, resulting in poor image quality and an inability to view the intended area.
There have been extensive studies of space-based radar for moving target imaging (MTI). Some of these studies include using STAP for ground clutter cancellation, as discussed above, and the use of Periodic Repetition Interval (PRI)-staggered post-Doppler filter banks. Ground moving target imaging (GMTI) is also known, as well as adaptive monopulse processing techniques based upon constraint optimization. However, the prior art does not discuss utilization of a variety of these techniques simultaneously, and is mainly focused on adaptive clutter cancellation for target detection and not for angle location purposes. Therefore there is a need to achieve both tracking and detection in a single unit.
One known technique for STAP optimization or clutter cancellation involves a tedious two-step procedure involving large size STAP optimization. The processing procedure initially involves an unconstrained adaptation of the sum beam. Secondly the difference beam is then adapted to minimize the output clutter. However, this difference beam clutter minimization is subject to the limitation that the monopulse values must be preserved at several target angles (constraint points), which results in increased utilization of resources (i.e. the adaptive degrees of freedom), and more importantly has been ineffective in producing and preserving a usable monopulse ratio. Moreover, this technique has not proven to be effective for the accommodation of mainlobe clutter cancellation.
It is an object of the present invention to provide a method for determination of a single sum and difference beam weight by taking data samples, determining the covariance matrices of the sum and difference beams and the matrices inverse then solving for the unit weight using the following equation   W  =                                          (                                          R                ΣΣ                            +                              R                ΔΔ                                      )                                -            1                          ⁢        δ                                          (                                          δ                H                            ⁢                              (                                                      R                    ΣΣ                                    +                                      R                    ΔΔ                                                  )                                      )                                -            1                          ⁢        δ              .  
It is a second object of the present invention to provide a method for determining a monopulse ratio using improved adaptive processing techniques which overcome the heretofore limitations on the utility of the monopulse ratio.
It is a further object of the present invention to provide a device capable of developing a clear clutter free radar image using the improved adaptive processing technique to determine a monopulse ratio which is sufficiently free from mainlobe clutter to enable both detecting and angle location determination while eliminating the tedious processes now commonly known in the art.
The monopulse ratio is the ratio of the overall difference beam to the overall sum beam of a radar array. To determine the ratio, an array must first receive a data signal. This data signal is initially processed to develop sum and difference beams. These sum and difference beams are filtered to localize the clutter, which is a portion of the received signal, resulting in final sum and difference beams for each velocity of data signal received by the array. These final sum and difference beams are then adaptively processed in accordance with this invention as follows.
First the sum and difference beams are deterministically beam formed. Secondly the beam formed sum and difference beams are related to a plurality of other deterministically beam formed sum and difference beams. Next the adaptive sum and difference weight is determined. This adaptive weight is then used to determine the overall sum and difference beams. It is these overall sum and difference beams which are used to determine the monopulse ratio, by dividing the difference beam by the sum beam. As a result of common factor cancellation due to the assumption of a single weight for both the sum and the difference beams a monopulse ratio can be preserved.
In the present invention there is also provided a method and apparatus for the cancellation of ground clutter to produce a high quality radar image that can be used for both detecting and tracking of moving objects. This is accomplished by a method for radar detection and tracking of a target using monopulse ratio processing. Initially, a received data signal is staggered to form a signal with a periodic repetition interval (PRI). The received signal is then filtered, and the clutter in the signal is localized. The filtered output is adaptively beam formed and processed to cancel the clutter in the filtered output, and form a final sub-array sum beam and final sub-array difference beam.
One feature of these applications is that the adaptive processing step can be performed using space-time adaptive processing. Additionally, the filter may be a Doppler filter derived using a Fast Fourier Transform. Further, tapering may be applied to reduce sidelobes when adaptively processing the sub-array sum and difference beams.
In yet another aspect there is provided a program storage device readable by a machine, tangibly embodying a program of instruction, executable by said machine to perform method steps for staggering PRI data, deriving a filtered output from a localized clutter signal obtained from the PRI data, adaptively forming a sub-array sum azimuth beam and a sub-array difference azimuth beam from the filtered output, canceling clutter in the filtered output using said sub-array sum beam and said sub-array difference azimuth beam, forming a final sum beam and final difference beam, wherein the final sum beam is used for target detection and the ratio of the final difference beam to the final sum beam is used for angle location determination.